Several interesting BSM particles do not decay to easily detectable final states, and an assessment of their observability requires dedicated studies. A clear case is the one of minimal WIMP Dark Matter (DM) candidates (see e.g. [1]} and references therein). The charged state in the DM electroweak multiplet is copiously produced, but it decays to the invisible DM plus a soft undetectable pion, owing to the small mass-splitting. WIMP DM can be studied at muon colliders in several channels (such as mono-photon) without directly observing the charged state [2]}, [3]}. Alternatively, one can instead exploit the disappearing tracks produced by the charged particle [4]}. The result is displayed on the left panel of Figure REF  for the simplest candidates, known as Higgsino and Wino. A 10 TeV muon collider reaches the “thermal” mass, marked with a dashed line, for which the observed relic abundance is obtained by thermal freeze out. Other minimal WIMP candidates become kinematically accessible at higher muon collider energies [2]}, [3]}. Muon colliders could actually even probe some of these candidates when they are above the kinematical threshold, by studying their indirect effects on high-energy SM processes [7]}, [8]}.
